Thank you for the additional information.
When the ContentUrl
property is set to point to an external URL (not from the application), it is not possible to access the document loaded inside the <iframe>. This is not restriction to the RadSplitter, but a general browser behavior to prevent cross server script attacks. If this is the case, there is no solution to this scenario.
If you have loaded a page from the same domain, where the page containing the splitter resides, you need to get reference to the splitter from the inner page, and resize it according to the page's size. For your convenience I have prepared a sample project implementing this approach.
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